Fiber-optic amplifiers for amplifying an optic signal comprising a four-port coupler are known, for example, from Auge J., et al, "Fortschritte im Bereich der optischen Verstarkung", Elektrisches Nachrichtenwesen, 4th Quarter 1992, pages 37 to 44. There a fiber-optic amplifier is shown which has two pump-light sources, two pump-light couplers, and a length of optical waveguide doped with erbium ions.
In fiber-optic transmission systems, fiber-optic amplifiers serve to amplify optical signals which are guided in the optical waveguides.
Through the pump light, the erbium ions contained in the length of optical waveguide, which are in an energetic ground state, are raised into an excited state, from which they revert, through either spontaneous or stimulated emission, to the ground state. The stimulated emission is caused by the optical signal travelling through the length of optical waveguide. If no optical signal is travelling through the length of optical waveguide, the transition to the ground state will be spontaneous, with the energy difference between ground state and excited state being converted into radiation which propagates along the length of optical waveguide.
During this propagation in the length of optical waveguide, the spontaneous radiation is amplified. The radiation resulting from amplified spontaneous emission (ASE) emerges at the two ends of the length of optical waveguide. This radiation will hereinafter be referred to as "ASE". In the case of erbium-doped fiber-optic amplifiers, the wavelength of this ASE is in the range around 1550 nm. It thus lies in the wavelength range of the optical signal.
In such transmission systems it is important to have information on the transmission link and on the operating states of the fiber-optic amplifiers in order to safeguard the transmission itself and ensure the necessary transmission quality.
This includes information as to whether input power is present for the fiber-optic amplifier and how high the output power of the latter is.
If no input power is present, this may point to a fiber break, for example. In that case it must be ensured that the transmission link is "eye-safe" i e that there is no potential hazard to the eye of a person looking, intentionally or unintentionally, into a broken optical waveguide. To guarantee this, the system or individual parts thereof must be shut down in the event of a fiber break.
From the above literature it is known to monitor the input and output power of a fiber-optic amplifier in order to control the amplifier's gain. Portions of the input and output power are coupled out by couplers forming part of the fiber-optic amplifier, and detected by photodiodes.
During the transmission of analog signals, such as TV signals, high signal power levels prevail. The coupled-out signal power can be detected with simple circuitry.
Things are different if the signal power is lower, as is the case in digital transmission systems, for example. The coupled-out signal power is correspondingly lower, and the amount of circuitry required to detect this low signal power increases.